Heretofore several different types of planar guiding structures have been suggested/investigated for the microwave and millimeter wave regions. These guides have varied in structure and operating principal, but each of these designs has a common feature that their critical dimensions are on the order of one half the guide wavelengths, .lambda..sub..epsilon. /2, or smaller. For the microwave and lower millimeter wave regions, this is, of course, advantageous because the guides have reasonable cross section dimensions, are easily fabricated by etching techniques and are well suited for the design of integrated circuits. A review of these guides may be found in such publications as: Antenna Handbook, by Y. T. Lo and S. W. Lee, Editors, Van Nostrand Reinhold, 1988, in particular, chapter 28 entitled, "Transmission Lines and Waveguides," by Y. C. Shih and T. Itoh; and Millimeter Wave Engineering and Applications, by P. Bhartia and I. J. Bahl, Wiley and Sons, New York, 1984, chapter 6.
In the upper millimeter and submillimeter wave regions, however, the guide dimensions become exceedingly small and the associated fight fabrication tolerances make these guides difficult and expensive to fabricate. This is because the guidance principle employed in dielectric guides is based upon the total reflection at the dielectric surface, which confines the transmitted energy to the interior of the guides. Typically, the width of these guides is chosen to be somewhat less than a half wavelength to avoid over-moding. Therefore, these small wavelengths in the upper millimeter and the submillimeter wave regions cause the guide width to become extremely narrow. This occurs especially when high-.epsilon. materials are used and therefore, these guides are very difficult to fabricate.
This problem is addressed by the present invention.